Two fixes in trace showing

[dwijnand]

First, summon Show[T] instances, when show = true, and fail inlining (i.e. compilation) if it doesn't exist.

Secondly, even when show = false (i.e. the default), use the Show instance if one exists.

The motivation here was I had some non-Showable value, for which there is a Show instance, that wasn't showing properly under show=true.

[SethTisue]

I'd be most comfortable approving this if you walked me through the changes at one of our sessions next week — probably it won't take long.

[dwijnand]

If show = true then instead of just supporting Showable, I actually summon the appropriate show instance and use it with the i interpolator. When show = false, I reach for a Show instance if one's available, before falling back to alwaysToString, as it used to do.

So now if the trace result is an (Type, Type) it will actually pretty print the types.